原式=(x^4-2x^2y^2+y^4)-4x^2y^2
=(x^2-y^2)^2-(2xy)^2
=(x^2+2xy-y^2)(x^2-2xy-y^2)
x^4-6x^2y^2+y^4
=(x^4-2x^2y^2+y^4) -4x^2y^2
=(x^2-y^2)^2-4x^2y^2
=(x^2-y^2+2xy)(x^2-y^2-2xy)
x^4-6x^2y^2+y^4=(x²-y²)²-(2xy)²=(x²-y²+2xy)(x²-y²-2xy)
(x^2-y^2+2xy)(x^2-y^2-2xy)
16x^4-6x^2y^2+y^4呢？
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